1. Field of the Invention
The present invention relates to a method for forming a porous film, and more specifically, a method for forming a porous film used in a semiconductor integrated circuit as a low-dielectric-constant film.
2. Background Art
In recent years, the speed of semiconductor devices has significantly increased, and concurrently, the problem of transmission delay due to a lowered signal propagation rate caused by wiring resistance and parasitic capacitance between wirings, or between wiring layers in multi-layer wiring portions has been growing. Since the wiring resistance elevates and parasitic capacitance increases with decrease in the wiring width and wiring distance accompanying the higher integration of semiconductor devices, such a problem tends to be increasingly significant.
Heretofore, in order to prevent signal delay due to increase in wiring resistance and parasitic capacitance, aluminum wirings have been substituted by copper wirings, and the use of materials for low-dielectric-constant insulating films as interlayer insulating films has been examined. For example, an SiOC (carbon doped silicon oxide) film formed by introducing methyl groups into an SiO2 (silicon oxide) film, or organic insulating materials such as the film of polyallyl ether derivatives have been known. However, since the dielectric constant of these films are 2.6 to 2.9, the lowering of the dielectric constant has been demanded for semiconductor devices in the generation of the design rules for further miniaturization.
On the other hand, a film having pores of a diameter of several to hundreds of angstroms therein, known as a porous low-k film, is promising as a material to fabricate a next generation of integrated circuits.
Conventionally, a porous low-k film has been formed by applying a polysiloxane resin composition containing a pore-forming agent (porogen) onto a semi conductor substrate, and performing heat treatment to the polysiloxane resin composition. The heat treatment accelerates the curing reaction of polysiloxane, and at the same time, decomposes and evaporates the pore-forming agent. Thereby, a large number of fine pores are formed in the insulating film. At this time, if the decomposition of the pore-forming agent becomes vigorous before the curing reaction of polysiloxane ends, the size of formed pores becomes small, and pore-forming rate (porosity) lowers. Therefore, a conventional method is designed so that the decomposition of the pore-forming agent becomes vigorous at a temperature higher than the curing temperature of polysiloxane, and specifically, the polysiloxane resin composition has been heat-treated at a temperature of 425° C. or above to form pores therein.
However, there has been a problem that heat treatment at such a high temperature causes the reliability of copper wirings to lower. Therefore, there is an urgent need to lower the heat-treatment temperature. In this case, it is necessary to lower the decomposition reaction of the pore-forming agent as well as to lower the curing temperature of polysiloxane. Even if the lowering of the curing temperature of polysiloxane is realized, a large quantity of the pore-forming agent remains in the film if the decomposition and vaporization of the pore-forming agent is insufficient, and the porosity is lowered and the desired dielectric constant cannot be achieved.